The opposite is also possible, of course.
We know that magical people are considerably longer lived and are harder to kill. It's also possible that they are somehow more resistent to cancer. Somehow, their cells are healthier, less likely to develop cancerous mutations, something.
So your character might be a rare case of cancer in the wizarding world, which would increase the stress of everyone involved since it's even less common then in the Muggle world. This would also force a reliance on more Muggle style or experimental techniques, since they have less experience with cancer.